Counterweights are commonly used in coordinate measuring machines and in a variety of machine tools to balance the weight of vertically movable machine elements. In a coordinate measuring machine a probe is attached to a vertically movable Z-rail which is also movable in a horizontal plane. Thus, the probe can be moved in three dimensions for contact with a workpiece which is being measured. The counterweight opposes the weight of the probe and the Z-rail so that they essentially float in the vertical or Z direction. The counterweight permits relatively fast vertical movement and reduces the required force for vertical drive.
Among the requirements of the counterweight are that friction be minimized as the movable element travels upward and downward. In addition, the counterweight must be relatively insensitive to misalignment. When the counterweight is slightly misaligned with the axis of movement, frictional forces should remain low.
A variety of counterweights are known in the prior art. A weight connected by a chain and a pulley to a movable element is disclosed in U.S. Pat. No. 1,782,573, Johnson. U.S. Pat. No. 3,671,133, Galbarini et al, discloses the use of a hydraulic piston connected by a piston rod and a system of flexible cables and pulleys to a machine tool. U.S. Pat. No. 3,580,133, Berthiex, discloses a counterweight system including a hydraulic balance compensating device and a system of cables and pulleys. U.S. Pat. No. 2,571,049, Meylich, discloses a pneumatic counterweight for a milling machine including an air cylinder, connecting rods and an air regulation system. In these systems friction is introduced by the pulley systems, and in hydraulic systems utilizing piston rods, misalignment of the cylinder with the axis of movement can cause increased friction and binding.
It is a general object of the present invention to provide an improved counterweight system for a machine.
It is another object of the present invention to provide a counterweight for a machine which is relatively insensitive to misalignment and which permits rapid movement.
It is a further object of the present invention to provide a counterweight system which is relatively free of frictional forces during movement of the element to which the counterweight is attached.
It is another object of the present invention to provide a counterweight system for a machine wherein the weight of the movable element is accurately balanced.
It is yet another object of the present invention to provide a counterweight system which is isolated from machine vibrations.